The present invention relates to a keyer system for electronic organs and other keyboard electronic musical instruments, and in particular to a keyer system comprised of a plurality of discrete keyer circuits which are connected in series in building block form so as to accommodate the entire keyboard.
In prior art electronic organs, sustain or percussive keying has been a continual problem from the standpoint of balancing performance and cost. This is due in great part to the necessity of a sustain system which requires elements which indirectly control the audio signal as compared to key switches which directly control it and are relatively low in cost. Furthermore, the elements traditionally used for sustain keying are quite large physically, are costly and require tuning and adjustment. In the past, diodes and neon lamps have been utilized to perform the indirect control element function, with electrolytic capacitors being used for envelope shaping.
With the rapid growth of solid-state electronics, field effect transistor keyers have been widely utilized to perform the indirect control element function, but the sustain system has still often required large external capacitors, with the concomitant problems of matching and balancing. Furthermore, a great number of integrated circuit pins have been required, usually many times greater than the customary maximum of forty pins per chip. This necessitates assembling a large number of integrated circuit chips thereby requiring a large printed circuit board or wiring area to make the necessary interconnections.
Because of the nature of the key switching system used to control the sustain system, slow attack has been very difficult to incorporate at a reasonable cost. Switching of sustain lengths has at times required the use of power transistor switches which adds cost and heat to the system.
Many of these problems have been overcome by the keyer system disclosed in co-pending application Ser. No. 892,385 filed Mar. 31, 1978 in the name of John W. Robinson wherein the charge on one capacitor is incrementally transferred to another capacitor and therefore to the keyer control terminal by rapidly and alternately switching a pair of FET's.
With a sixty-one key manual, and even with smaller manuals such as those having forty-four keys, a great deal of wiring or printed circuit board is required for interconnecting the large number of keyers which are required. This adds cost to the system and makes servicing and maintenance of the circuitry difficult. Even where a number of keyers are contained within a single integrated circuit chip, the limitation of having forty or less external pins for controlling the chip and providing the necessary inputs and outputs for the tones and keydown data is a problem without sacrificing flexibility and features of the organ.